1. Field of the Invention
The present invention is related to device characterization methods and circuits, and more particularly to delay-based techniques for characterizing bias temperature instability effects.
2. Description of Related Art
As geometry and power supply voltages in very large-scale integrated circuits (VLSI) such as semiconductor memories and microprocessors are decreased, the effect of threshold voltage variation has become increasingly significant. Not only do process variation changes in threshold voltage cause variation from device-to-device, but effects such as negative bias temperature instability (NBTI) and positive bias temperature instability (PBTI) cause changes in performance that are time and stress dependent. The mechanisms behind NBTI and PBTI, referred to generally as bias temperature instability (BTI) are not fully understood, and measurements of their effects have been limited by their time-dependent nature, particularly due to the fast partial recovery of observed threshold voltage shifts due to BTI after stress is removed.
NBTI effects are seen when a negative gate voltage stress is applied to a P-channel metal-oxide semiconductor (MOS) transistor, and the effects diminish rapidly during the recovery time immediately following the removal of the stress. Similarly, PBTI effects are seen in N-channel MOS devices, particularly in those with high-k gate dielectrics. Therefore, in order to properly characterize BTI effects, in particular to simulate aging by applying a stress and measuring a change in threshold voltage before recovery, and also to gain insight into the mechanisms causing BTI, it is desirable to measure threshold voltage not only during the application of the stress and immediately after removal of the stress, but to characterize the entire transient threshold voltage recovery evolution after stress.
Present BTI measurement techniques provide threshold voltage recovery observation on the order of microseconds and later. Some techniques directly measure a threshold voltage change during BTI recovery by observing voltages a terminals of one or more transistors to which a stress has been previously applied, while others use techniques such as ring oscillator measurements that measure a beat frequency between a ring oscillator having stressed devices and a ring oscillator having un-stressed devices. However, existing techniques do not provide a sufficiently high resolution with respect to the recovery time to permit the BTI recovery to be characterized in the sub-microsecond range or to permit characterization of changes in recovery during repetitive stress applications at rates on the order of microseconds or faster. Such repetitive stress application is highly desirable for characterizing the long-term aging effects of BTI. Further, some of the existing techniques fail to isolate only one type of BTI effect (NBTI or PBTI without other effects such as Hot Carrier Injection), and also may fail to eliminate other factors in the measurement process caused by the application of stress.
Therefore, it would be desirable to provide methods, circuits and systems for BTI characterization that measures recovery characteristics from BTI effects in the sub-microsecond region, as well as the effects of continuous stress experiments while minimizing the unwanted threshold voltage recovery when stress conditions are temporarily removed to perform each measurement. It would further be desirable to provide such BTI characterization that measures the BTI effects after repetitive applications of stress, i.e. AC stress, at repetition periods of a microsecond and faster.